Unclear assignment of root in dependency_graph.add_child_vertex

[baron1405]

In the dependency_graph.add_child_vertex the variable root is assigned:

root = !parent_names.delete(nil) { true }

This assignment does not makes sense to me.

Test Case	root
![].delete(nil) { true }	false
![nil].delete(nil) { true }	true
!["a", "b"].delete(nil) { true }	false
!["a", "b", nil].delete(nil) { true }	true
![nil, "a", "b"].delete(nil) { true }	true

If the intent is to allow a root vertex to be created using the add_child_vertex method, the criteria is that the parent_names list is empty (or for some reason only contains nil values). In general, it seems like creating a root vertex in this method is undesirable and an exception should be thrown if the parent names list is empty. In any case, the current implementation is either incorrect or a comment should be added to clarify the intent.